使用阵列列表平衡二进制搜索树

Question

对于类分配，我需要在提供的binarysearchtree类中添加一种方法，该类将通过存储数组中的dropect in andray和使用这些值来平衡二进制搜索树构造一棵新树。但是，当我尝试运行该方法时，我会得到NullPoInterException。如何更改我的方法以正确平衡我的二进制搜索树？

我在下面包含了我的代码（试图将其简化为问题所需的内容）；底部的两种方法是我试图用于平衡的方法。

package ch08;
import ch05.queues.*;
import java.util.ArrayList;
public class BinarySearchTree<T extends Comparable<T>> implements BSTInterface<T>{
protected BSTNode<T> root;      // reference to the root of this BST
protected LinkedUnbndQueue<T> inOrderQueue; 
protected ArrayList<T> balanceArray;
public BinarySearchTree(){
    root = null;
}
public int reset(int orderType){
    int numNodes = size();
    if (orderType == INORDER){
        inOrderQueue = new LinkedUnbndQueue<T>();
        inOrder(root);
    }
    return numNodes;
}
public T getNext (int orderType){
    if (orderType == INORDER)
        return inOrderQueue.dequeue();
}
    public void balanceTree() {
    int count = reset(INORDER);
    for(int i = 0; i < count; i++) {
        balanceArray.add(getNext(INORDER));
    }
    BinarySearchTree<T> tree = new BinarySearchTree<T>();
    tree.insertTree(0, count - 1);
    this.root = tree.root;
}
public void insertTree(int low, int high){
    if(low == high) {
        add(balanceArray.get(low));
    }
    else if((low + 1) == high) {
        add(balanceArray.get(low));
        add(balanceArray.get(high));
    }
    else {
        int mid = (low + high) / 2;
        add(balanceArray.get(mid));
        insertTree(low, mid - 1);
        insertTree(mid + 1, high);
    }
 }
}

Answer 1

我猜nullpoInterException来自以下事实：您永远不会初始化balanceArray。

您不应将inOrderQueue和balanceArray声明为字段（IMHO），因为它们仅与一个操作有关。我会在那里使用参数。

我看不到类BSTNode，所以我认为这是这样的：

public class BSTNode<T extends Comparable<T>> {
    T value;
    BSTNode<T> left;
    BSTNode<T> right;
    public BSTNode(T value, BSTNode<T> left, BSTNode<T> right) {
        this.value = value;
        this.left = left;
        this.right = right;
    }
}

这是我要进行平衡的方式：

public class BinarySearchTree<T extends Comparable<T>> {
    protected BSTNode<T> root;      // reference to the root of this BST
    public BinarySearchTree() {
        root = null;
    }
    // creates a tree from a sorted list
    private BinarySearchTree(List<T> list) {
        root = buildTree(list, 0, list.size());
    }
    public BinarySearchTree<T> balancedTree()  {
        List<T> list = new ArrayList<>();
        inOrder(root, list);
        return new BinarySearchTree(list);
    }
    private void inOrder(BSTNode<T> node, List<T> list) {
        if (node != null) {
            inOrder(node.left, list);
            list.add(node.value);
            inOrder(node.right, list);            
        }
    }
    private BSTNode<T> buildTree(List<T> list, int i, int size) {
        if (size == 0) {
            return null;
        } else {
            int half = size/2;
            int mid = i+half;
            return new BSTNode<T>(list.get(mid),
                    buildTree(list, i, half),
                    buildTree(list, mid+1, size-half-1));
        }
    }
    ...
}